Rechargeable charge storage arrangements, like accumulators, play an important role in electric vehicles, like electric cars, but also in other mobile electric devices, such as computers. Such charge storage arrangements usually include a plurality of charge storage cells (accumulator cells) connected in series, with the number of cells connected in series being dependent on the supply voltages provided by the individual charge storage cells and being dependent on the desired overall supply voltage to be provided to a load. The supply voltage provided by a storage arrangement including a number of storage cells connected in series corresponds to the sum of the individual supply voltages of the storage cells.
The rechargeable storage arrangement is discharged when it powers the load, and it can be recharged when an external power supply is available. Most types of charge storage cells, such as lithium-ion cells, should not be charged so as to have a supply voltage which is above a given upper voltage limit and should not be discharged so as to have a supply voltage which is below a given lower voltage limit. Otherwise there is the risk of damage or degradation. Due to inevitable variations in the manufacturing process of the storage cells, the capacitances of the individual storage cells may slightly differ from one another, i.e., the capacitances may be “unbalanced”. This unbalance may cause some of the storage cells to reach the upper voltage limit during the charging process earlier than other storage cells, and may cause some of the storage cells to reach the lower voltage limit during the discharging process (when a load is powered) earlier than other storage cells. When one of the cells reaches the upper voltage limit the charging process has to be stopped, even if the other cells have not completely been charged yet, and when one of the cells reaches the lower voltage limit the discharging process has to be stopped, even if other cells have not been discharged to their lower limit.
Charge balancing circuits monitor the charging state of the individual storage cells and are configured to selectively charge or discharge individual cells in order to balance the charging states of the individual cells and to provide for an increased efficiency in use of the storage arrangement.
A charge balancing circuit according to a known concept includes inductive storage elements which are inductively coupled. One (or more) of these inductive storage elements is configured to be connected in parallel with the overall arrangement, and the others are configured to be connected in parallel with the individual storage cells. Electronic switches can be used to connect/disconnect the inductive elements to/from the storage arrangement or the storage cells, respectively. By virtue of this arrangement energy can be transferred from one storage cell to the overall arrangement (or parts of it), or from the overall arrangement to one storage cell.
Errors may occur during the operation of such a balancing circuit. These errors include a missing inductive coupling between the individual inductive elements, a defect in one of the inductive elements, or a defect in one of the switches. There is, therefore, a need to detect such errors.